The effect of microstructure on hardness and toughness of low carbon welded steel using inert gas welding PDF

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Materials and Design 32 (2011) 2042–2048 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes The effect of microstructure on hardness and toughness of low carbon welded steel using inert gas welding Ehsan Gharibshahiyan a,⇑, Abbas Honarbakh...

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The effect of microstructure on hardness and toughness of low carbon welded steel using inert gas welding ehsan malek Materials & Design

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Materials and Design 32 (2011) 2042–2048

Contents lists available at ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

The effect of microstructure on hardness and toughness of low carbon welded steel using inert gas welding Ehsan Gharibshahiyan a,⇑, Abbas Honarbakhsh Raouf b, Nader Parvin c, Mehdi Rahimian d,1 a

R&D Department, Oghab Afshan Industrial & Manufacturing Company, Semnan, Iran Engineering Faculty, Materials Department Semnan University, Damghan Rd. km 3, Semnan, Iran c Faculty of Mining and Materials Engineering, Amirkabir University of Technology (AUT), Hafez Ave., Tehran, Iran d Faculty of Engineering, Islamic Azad University – Semnan Branch, Semnan, Iran b

a r t i c l e

i n f o

Article history: Received 26 August 2010 Accepted 24 November 2010 Available online 3 December 2010 Keywords: Welding Microstructure Brittle fracture

a b s t r a c t The heat affected zone (HAZ), has a great influence on the properties of welded joints since it can alter the microstructure and residual stresses. In this paper, the effect of welding parameters and heat input on the HAZ and grain growth has been investigated. The role of grain size on hardness and toughness of low carbon steel has also been studied. It was observed that, at high heat input, coarse grains appear in the HAZ which results in lower hardness values in this zone. For example raising the voltage from 20 to 30 V decreased the grain size number from 12.4 to 9.8 and hardness decreased from 160 to 148 HBN. High heat input and low cooling rates produced fine austenite grains, resulting in the formation of fine grained polygonal ferrites at ambient temperature. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Welded joints are heterogeneous by nature and present a gradient of properties in the base metal and the HAZ. The welded joints can be classified on the basis of coherency between the welded zone and the base metal [1]. The control of microstructure is of prime importance in such area. One of the major factors affecting the toughness of the welded metal is the formation of local brittle zone (LBZ). The degree of brittleness in this zone varies with material chemistry and welding conditions [2]. It has been reported that the formation of more than 77% needle ferrite phase, led to a reduction in toughness in steel [3]. Furthermore, the grains close to the fusion line, behave as the origins of nucleation. Since the weld pool easily wets the substrate, crystals nucleate upon the substrate grains. Epitaxial nucleation and growth proceed without altering the crystallographic orientations. As solidification proceeds, grains tend to have columnar growth, perpendicular to the pool boundary since maximum temperature gradient exists there [4–5]. However, the elevation of heat input and welding speed, can lead to the formation of

⇑ Corresponding author. Address: No. 64, Abdos alley, Post 2 Street, Chamran Blvd, Semnan, Postal code: 3519756561, Iran. Mobile: +98 9121317822. E-mail addresses: [email protected] (E. Gharibshahiyan), [email protected] (A.H. Raouf), [email protected] (N. Parvin), [email protected] (M. Rahimian). 1 Address: No. 87-1, Shohada 22, Shohada Street, Semnan, Postal code: 3313645575. Iran. Mobile: +98 9125322792. 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.11.056

equiaxed grains [6–9]. Balasubramanian and coworkers [10] studied the properties of welded AA 6061 Aluminum and reported that the tensile strength is directly proportional to the existence of equiaxed grains in the microstructure. This phenomenon has been attributed to the reduced sensitivity of finer grains to solidification cracks. Secondly they contribute to enhanced fracture toughness and ductility. Generally, high heat input and low cooling rate accelerate nucleation and recrystallisation and hence grain growth in the HAZ. Several grain refining techniques have emerged to counteract this effect, namely weld pool stirring, arc oscillation and arc pulsation [11]. This paper deals with the effect of welding parameters on grain size and phase conditions in low carbon steel.

2. Experimental procedure The chemical composition and mechanical properties of the test specimens are shown in Table 1, and that of the welding electrode are given in Table 2. The dimensions of Charpy impact test specimens were 10  10  55 mm and had a V-notch of 1.6 mm wide and 5 mm deep. All impact tests were carried out according to ASTM E23 standard [12]. A ParsMIG-SP501 welding machine with a shielding gas of CO2 and the electrode was a standard AWS-ER 70S-6 having a diameter of 1 mm. The welding parameters are given in Table 3. The specimens were polished and etched in 2% Nital to reveal the microstructure [13,14]. The microstructure photos were taken

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E. Gharibshahiyan et al. / Materials and Design 32 (2011) 2042–2048 Table 1 Chemical composition and mechanical properties of base metal. % Si

%S

%P

% Mn

Yield strength (MPa)

Ultimate strength (MPa)

Elongation (%)

0.1

0.016

0.009

0.508

436

477

29

Table 2 Chemical composition and mechanical properties of filler metal. %C

% Si

%S

%P

% Mn

Yield strength (MPa)

Ultimate strength (MPa)

Impact of energy (j)

Elongation (%)

0.06

0.84

0.008

0.009

1.38

489

578

105

7

Table 3 Welding parameters. Filler wire diameter (mm) Voltage (V) Amperage (A) Welding speed (cm/min) Gas flow rate (l/min) Shielding gas

1 20, 25, 30 130, 180 40 12–17 CO2

from these etched surfaces by means of Nikon Stereo Zoom optical microscope. A Philips XL30 scanning electron microscope (SEM) was used for examination of fracture surfaces of weld metal. Clemex image software was employed for microstructural analyses and Brinell hardness tests were performed at a load of 125 kg on polished surfaces [15].

3. Results and discussion 3.1. Microstructure The microstructure of the HAZ depends on chemical composition and the peak welding temperature and welding voltage [16]. Fig. 1 illustrates the microstructure of the specimens welded at 20 and 30 V, showing larger grains at high voltage conditions. The energy transfer per unit length of weld is a function of heat input, which in turn can alter the mechanical properties of the welded zone. The heat input can be calculated as follows [17]:

H¼

60  E  I 1000  S

ð1Þ

where H is the heat input (kJ/mm); E the voltage (V); I the current (A); and S is the travel speed (mm/min). The effects of welding parameters on the properties are shown in Fig. 2. It demonstrates that high heat input and low cooling rates widen the HAZ and increase the grain size. The cooling rate is of prime importance. Low cooling rate entails an increase in the size of austenite grains. The mutual dependency of voltage and heat input is shown in Eq. (1). Grain boundary migration and mean grain diameter (D) enlargement occur following a high heat input. On the other hand, grain boundary migration energy (c) and grain size are related by Eq. (2). It is expected that at low welding speeds the grain boundary energy increases leading to grain growth [18].

dD 2c ffi dt D

Fig. 1. Optical micrographs: (a) microstructure of welded specimen with 20 V and (b) microstructure of welded specimen with 30 V.

ð2Þ

Fig. 3 illustrates the microstructure and grain size both in the welded metal and also in the heat affected zone. The interrelationship of voltage and retention time of temperature above the recrystallisation temperature can be justified.

Fig. 2. Effect of heat input per unit length of weld on: (a) width of HAZ (shaded), (b) thermal cycles near fusion boundary, and (c) strength or hardness profiles.

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Fig. 3. Optical micrographs (a–c) for specimen with voltage 20, 25, 30 V, (d–f) for heat affected zone for the same specimens.

The grain size number was calculated according to ASTM E 11296 [19]. Grain size number is related to grain size according to Eq. (3).

N ¼ 2n1

ð3Þ

where N is the grain size number and n is the grain size and are inversely proportional to each other. As the grain size increases, the number of grains in one square inch drops. A comparison of grain sizes in welded metal are illustrated in Fig. 4, indicating lower grain size in the former. The grain size number in the welded metal was 12.4 at 20 V as compared to that of 9.8 at 30 V. Welding is a dynamic process in which the heat source is in at a state of constant motion, leading to temperature gradient variations. It has been well established that initiation of solidification will not proceed unless the grain radius of the solidifying grain

(r) is larger than the critical nucleation radius (r), which in turn is related to inverse of supercooling [18].

r ¼

  2csl T m 1 Lv DT

ð4Þ

where csl is the solid/liquid interfacial free energy, Lv is the latent heat of fusion per unit volume, Tm is the melting point and DT is supercooling [18]. The formation of solid particles, necessitates that nuclei energy be higher than the critical nucleation energy barrier, and is inversely proportional to the square of supercooling.

DG /

1 DT 2

ð5Þ

As can be seen, at high supercooling levels, r  and DG are low. The more supercooling, the less is the grain size. The arc speed V, is related to grain growth rate (R) according to the formula:

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Fig. 4. Comparison of the grain size in the WM: (a) welding voltage 20 V and (b) welding voltage 30 V.

R ¼ V  cosðhÞ

ð6Þ

It can be deduced from Eq. (5) that the rate of solidification is greatest at the weld centerline. High heat input together with low temperature gradient led to a higher constitutional supercooling. Thus, high rate of solidification coupled with high heat input can result in a finer grain sizes (Fig. 5). Grain coarsening in the HAZ can be explained by the operating thermal cycle and diffusion. The grains close to the fusion line are exposed to higher temperatures, therefore grain growth occurs. Clearly large ferrite grains result in large austenite grains. Another important factor that contributes to grain coarsening is grain boundary mobility and fusion. Grain boundary movement depends on diffusion and atomic migration on both sides of grain boundary. Diffusion itself is a function of retention time at high temperature and leads to the migration of atoms and displacement of grain boundaries. The relation between atomic displacement and time is given by the equation:

pffiffiffiffiffiffi r ¼ 2:4 Dt

ð7Þ

r, is the radial distance from the origin. D, is the diffusion coefficient which is related to the temperature, and t, is the elapsed time. It can be seen that atomic displacement is directly proportional to the square root of time. As mentioned earlier, low welding speed and high heat input promote large grains. This can be verified by comparing the surface area of grains which exist in different welding zones. Due to larger grains, the number of grains in the HAZ is low. Table 4 gives the area and number of grains in both HAZ and welded metal. Besides voltage, the current contributes to grain size as well. For instance, increasing the current from 130 to 180 A, led to grain refinement in the welded metal. As illustrated in Fig. 6, rapid cooling prevented the growth of grains and the initial grain size is maintained after cooling. It was observed that the grain size number was 10.6 at180 A, as compared to that of 9.8 at 130 A. 3.2. Growth of pearlite It was observed that heat input and the amount of pearlite were inversely proportional to each other. For instance, the amount of

Table 4 Variety of number and area grains in the WM and HAZ.

Fig. 5. Nucleation and formation of equiaxed grains in weld metal.

Voltage (V)

Grain size number

Max. area (lm2)

Min. area (lm2)

Ave. area (lm2)

WM

20 25 30

288 63 46

221 420 691.7

0.3 1.7 9.6

28.5 116.7 217.7

HAZ

20 25 30

69 59 32

1897 2107 5882

10.9 39.7 9

397.6 436.9 607

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Fig. 6. Comparison of the grain size in the WM: (a) welding current 130 A and (b) welding current 180 A.

pearlite was reduced from 17.8% to 7.4% at heat inputs of 5.4 and 8.1 kJ/cm respectively (Fig. 7).

The rate of pearlite formation can be varied under certain circumstances. An important factor is the amount of carbon diffusing in the metal and its effect of pearlite layers formation. Diffusion is directly proportional to the temperature according to following equation

D ¼ D0 exp



Q RT



ð8Þ

where D is the diffusion coefficient, D0 is the self diffusion, Q is the activation energy, R is the Boltzmann’s constant and T is the absolute temperature. During cooling, austenite to pearlite transformation proceeds at a fast rate due to its nucleation and growth. If a specimen undergoes a low heat input, there would be a suitable condition for nucleation in the, edges, and grain boundaries. Consequently, carbon can dissipate extensively resulting in the formation of pearlites. However, supercooling from a high temperature, may contribute to reduced nucleation and hence the formation of ferrites. At high supercooling conditions, there would be a greater tendency for the formation of ferrites from the grain boundaries, possibly in the form of widmanstatten. 3.3. Hardness

Fig. 7. Percentage of pearlite and ferrite phase in the HAZ: (a) heat input 5.4 kJ/cm and (b) heat input 8.1 kJ/cm.

Any increase in the heat input has an inverse effect on the hardness of welded metal and it’s HAZ. As an example, elevation of heat input from 5 to 8 kJ/cm reduced the hardness from 160 to 148 HB near the HAZ whereas hardness values of 195 and 205 HB were measured in the corresponding welded metal. With due regards to Fig. 8 the above mentioned phenomenon can be explained. One of the factors contributing to lower hardness in the HAZ zone is that of high heat input and hence retention of heat in this region.

E. Gharibshahiyan et al. / Materials and Design 32 (2011) 2042–2048

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Fig. 8. Hardness profile of welded specimens.

Generally, grain nucleation and growth of austenite; can lead to reduced dislocations and work hardening compared to its elementary condition. Annealing of the HAZ, can have pronounced effect on phase content and morphology. The net effect is reduced dislocations and hardness. In spite of the presence of high temperature in the welded metal, the cooling rate is also high which presents the nucleation of very fine grains. Besides, fine grain structures exhibit low intergranular spacing. The stress for dislocations to cross grains can be calculated using the following relationship [20]:

s0 ¼

Gb k

ð9Þ

where G is the shear Young modulus and b, is the dislocation Berger’s vector. High hardness in the welded zone may be attributed to fine grain size, needle shaped ferrite or the existence of widmanstatten inside ferrite grains. Hardness and grain size are inversely proportional [21] as given by following equation

K H ¼ H0 þ pffiffiffi d

ð10Þ

where H is hardness, K is proportionality constant and d is grain size. Thus the lower hardness in the HAZ may be related to grain growth and the existence of ferrite phase in this region, which has been reported by previous authors [1,16]. 3.4. Toughness Absorption of impact energy can be controlled by chemical composition, microstructure and the heating cycle [2]. The range of impact strength values of 92.3 and 40.8 kJ was achieved after welding at 130 A/20 V, and 180 A/30 V respectively. A downward trend in the impact energy was observed at 130 A and variable voltages (Fig. 9). Excess grain growth can lead to reduced strength and increase crack initiation and growth. Furthermore, it can adversely affect the fracture toughness, which may arise due to heating and cooling cycles. As seen in Fig. 10 weld metals showed a predominantly ductile fracture appearance of microvoid coalescence (MVC) with heat input of 5.4 kJ/cm. However, at 6.75 kJ/cm, entirely ductile fracture

Fig. 9. Absorbed charpy impact energy of weld metal.

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lower than that of the pearlite, crack initiation and growth is more likely to occur in the ferrite grains. On the other hand, dislocations can move easier in structures containing large grains resulting in reduced ductility. 4. Conclusion The effect of heat input on microstructure and mechanical properties of low carbon welded steel was investigated. Results showed that by raising the voltage from 20 to 30 V the grain size number decreased from 12.4 to 9.8. It was also observed that high heat input and rapid cooling rates, in the weld metal produced fine grain austenite at high temperature, results in the formation of fine grained polygonal ferrites at ambient temperature. High heat input led to grain coarsening which was more pronounced in the HAZ, as well as reducing the impact energy and toughness. Elevation of heat input reduced the hardness in the HAZ, for instance, raising the heat input from 5 to 8 kJ/cm decreased the hardness from 160 to 148 HBN. This is considered to be attributed to a reduction in the density of dislocations and microstructural coarsening. Acknowledgement The authors would like to thank from managing director of Oghab Afshan Industrial & Manufacturing Company for financial support of this study. References

Fig. 10. Fracture surface of weld metal: (a) 5.1 kJ/cm, (b) 6.75 kJ/cm, (c) 8.1 kJ/cm.

together with local cleavage-type fracture appeared. The fractured surfaces with 6.75 kJ/cm showed a combination of transgranular fracture and microvoid coalescence (MVC). The fracture surface of weld metal broken with heat input 8.1 kJ/cm exhibited a brittle fracture behavior. Due to high heat input, transgranular fracture by quasi-cleavage and relatively flat surface was observed. At low cooling rates, growth of austenite grains proceed, which ultimately lead to the formation of coarse pearlite and planar ferrite. Existence of acicular ferrite ...